Patent GB-748783-A (GB)
IPC: H04W84/02
Abstract: 748,783. Automatic exchange systems. GENERAL ELECTRIC CO., Ltd., and FAIRBAIRN, E. P. June 2, 1954 [June 12, 1953], No. 16229/53. Class 40 (4). A group of radio substations works to a main station which is connected to an automatic exchange. The substation can make revertive calls through the exchange switches. The main radio station transmits a carrier F2 using a transmitter TX. The substations each transmit a carrier F1 when calling and F4 when answering. The main station includes corresponding receivers RC, RA. To call a substation, a generator 6 modulates the common carrier F2 with a frequency characteristic of the called substation. The main station has access to the exchange selectors over line wires 7, 8 and is accessible from a final selector connected to line wires 15, 16. Relays TA, TB ... TK are individual to the substations and when operated they enable the generator 6 to apply the corresponding characteristic calling frequency to transmitter TX. Another calling frequency f3 is transmitted over the exchange line 7, 8 by the generator 6 to signal a revertive call. In a call from the exchange to a radio substation, the final selector is set in the usual manner on the terminals of the wanted substation, say the station identified by TC. Ground on the P-wire is extended over lead 3 to start the transmitter TX. Relays TC, D pull up in the loop and at contact tc3 the appropriate calling frequency is selected and is generated by 6 under the control of interrupted earth on terminal 17. Relay D returns ringing tone to the calling line through transformer I. When the called substation replies with carrier F4, receiver PA responds and relay AA connects the calling line to the transmitter over leads 1, 2 and a resistance network, ringing tone being cut off at the same time. Relay E pulls up, locks to the P-wire and stops the generator 6. When the calling party hangs up, the exchange switches release in the normal manner and the relays TC, D fall back. The substation continues to be marked busy on the P-wire until the substation stops transmitting F4 and relay AA falls back. In a call from a radio substation, receiver RC responds to the carrier F1 and relay AC connects the main station to the exchange line wires 7, 8. Relay B follows and impulses in the carrier F1, or in modulations thereon, are repeated over line wires 7, 8 to operate the exchange switches in the normal manner. The transmitter TX is started when ground is returned on the P-wire. A dialling relay C pulls up at the beginning of every impulse train. With relay C up, generator 6 applies a frequency f3 to a phantom circuit over the exchange line wires 7, 8, but this has no effect unless the call is revertive. While relay C is up, there is busy ground on the P-wire 25 only when AC is back during the impulses. If the final selector tests free whilst both C, AC are up and is set on the terminals of a substation, busy tone is returned over the line wires 15, 16 from transformer I. If the call is revertive, the find selector is set on the terminals of the station identified by, say TB, the final selector being marked free on the P-wire during the release period of C when impulsing stops. A device 10 responds to the frequency f3 and relay R pulls up with TB and D. Relay X locks, transmits ringing tone to the calling substation through the exchange switches and the transmitter TX which was started by ground on the P-wire 25 at the final selector. In TX, the carrier F2 is modulated with the appropriate calling frequency. On reply on carrier F4, relay AA pulls up as before, relay E follows and the line wires 7, 8 are disconnected. The exchange switches release but the call proceeds with relays AA, AC, E, B, X operated, the final selector being busied by ground on the P-wire 25. The transmitter TX and the receivers RA, RC are permanently interconnected by a resistance network. Different carrier frequencies are used by other main radio stations, but the same calling frequencies may be used in each group of stations.